The present invention relates generally to vibration damping coatings, particularly for use on structural components of gas turbine engines subject to vibratory energy.
In gas turbine engines, there are a number of rotating and fixed structural components subject to vibratory energy. Components subject to vibratory energy include blades, vanes, and foils. The components are generally beam-like structures, often cantilevered, that are subject to natural frequencies of vibrations, or resonant frequencies. The natural frequencies of vibration, or resonant frequencies are excited through mechanisms, such as mechanical vibration and fluid flow. Natural frequencies are frequencies at which an ideal system will vibrate with zero input excitation power. In a real system there exists a certain amount of intrinsic or added damping. The real system will respond at the natural frequencies and displacement amplitude will grow to the point that damping dominates or until the part fails. Damping is the conversion of mechanical energy to heat.
Rotating components such as fan rotor blades or blisks are prone to vibration at certain speeds. Fan rotor blades are blades that are fastened to a center mounting. Fan rotor blades have the advantage that individual blades may be removed, repaired and/or replaced. A blisk is a single-piece component, consisting of a disk and blades. Blisks are also known as integrally bladed rotors or IBRs. Blisks have the advantage over the conventional disk and blade arrangement of potential weight saving through the elimination of the mountings that secure the blade root to the disk. However, like the fan rotor blades, vibration leads to fatigue and eventually to pre-mature, and often catastrophic, failure of the component.
Of the vibrating components of the gas turbine engine, the rotating components are under the most stress and are the most difficult to treat due, in large part, to the combined effects of mechanical and fluid dynamics, the latter of which is associated with fluid turbulence.
Vibration originates from a variety of sources. For example, one source of vibration energy in fan rotor blades or blisks is mechanical imbalance. Another source of vibration energy is fluid dynamic loading. Fluid dynamic loading is a result of vortex shedding at the trailing edge of a rotating blade. If one or more natural frequencies of the blade lie within the vortex shedding frequencies, then the blade will be excited into motion, and begin vibrating. Damping can be used to reduce the amount of vibration.
For fan blades and stator vanes, previous damping treatments have most often been applied at the base of the components, where they attach to the rest of the machine, at the tip in the form of a shroud for the blades, and at the inner and outer shroud for vanes. Damping at the blade tip by a shroud is effective in reducing the dynamic vibration levels of cantilevered blades, but has the drawback of increased weight and centrifugal forces imposed on the blades and the rotor hub. Intermediate damping positions have been used in the form of extensions normal to the blade that are positioned between the blades at locations part way between the blade root and tip. The extensions normal to the blade have the drawback that they impose extra weight, and disturb the fluid flow around the appendage, which reduces the efficiency of the engine. Another attempt to reduce vibration included friction devices mounted at the connections between the blade and the hub. These friction devices rely on the relative motion between the blade base and the hub. Vibrational energy is extracted from the blade and converted to heat. This approach has the drawback that the motion of the blade is low at the junction between the blade and the hub. Additionally, this approach is only effective when the friction devices are placed at locations of large displacement.
Another approach for reducing vibration includes dynamic absorbers. Dynamic absorbers reduce vibration levels in many types of devices. In one application, a liquid is placed within a chamber of a hollow blade. The liquid oscillates within the chamber, which is sized to produce a resonant frequency approximately the same as that of a dominant resonance in the blade. The combination of the blade resonance and the fluid resonance form a system in which energy from the blade, which has low intrinsic damping is coupled to energy in the liquid, which through proper selection of viscosity, has high intrinsic damping. This approach has the drawback that the dynamic absorber formed by the liquid oscillator only extracts energy from the blade in a relatively narrow band of frequencies. Since the excitation mechanism is typically a larger band of frequencies then a narrowband absorber, the dynamic absorber will only provide partial vibrational damping.
In still another approach, treatment of vibrations have included hollowing out the blade structure and filling the void with a high-density granular fill, such as sand or lead shot, or a low-density material, such as low-density polymer or ceramic. Broadband treatment has been achieved by filling hollow shafts with sand, but the enhanced performance comes at the cost of a substantial weight increase that is unsuitable for many applications.
Accordingly, what is needed is a method for damping that avoids the mechanical and manufacturing disadvantages encountered in the prior art discussed above, while still providing damping effect that increases the life and structural integrity of components subject to vibrational energy.